Подскажите, почему в этом классе: 
static abstract class SeaCreature {
        public void swim() {
            SeaCreature currentCreature = (SeaCreature) getCurrentCreature();
            currentCreature.displaySwim();
        }

        private void displaySwim() {
            System.out.println(getCurrentCreature().getClass().getSimpleName() + " is swimming");
        }

        abstract CanSwim getCurrentCreature();
    }
этот метод: 
abstract CanSwim getCurrentCreature();
типа CanSwim а не SeaCreature ? ниже представлен полный код: 
public class Solution {
    public static void main(String[] args) {
        CanSwim creature = new Orca();
        creature.swim();
        creature = new Whale();
        creature.swim();
        creature = new RiverOtter();
        creature.swim();
    }

    public static void test(CanSwim creature) {
        creature.swim();
    }

    interface CanWalk {
        void walk();
    }

    interface CanSwim {
        void swim();
    }

    static abstract class SeaCreature {
        public void swim() {
            SeaCreature currentCreature = (SeaCreature) getCurrentCreature();
            currentCreature.displaySwim();
        }

        private void displaySwim() {
            System.out.println(getCurrentCreature().getClass().getSimpleName() + " is swimming");
        }

        abstract CanSwim getCurrentCreature();
    }

    static class Orca extends SeaCreature implements CanSwim {
        public CanSwim getCurrentCreature() {
            return this;
        }
    }

    static class Whale extends SeaCreature implements CanSwim {
         public CanSwim getCurrentCreature() {
            return this;
        }

    }

    static class RiverOtter implements CanSwim, CanWalk {
      public void walk() {

       }
      public  void swim() {

       }

    }
}